The present invention relates to a difluoromethyl ether derivative which is useful as a liquid crystalline compound, a liquid crystal composition comprising the compound and a liquid crystal display element using the composition. Further, the present invention relates to a simple and efficient process for producing the difluoromethyl ether derivative.
A liquid crystal display element makes use of optical anisotropy and dielectric anisotropy of a liquid crystal crystalline compound and is used for watches, electric calculators, various measuring instruments, panels for automobiles, word processors, electronic notebooks, printers, personal computers, televisions and portable telephones. There has been an increasing demand therefor year by year. A liquid crystal phase is between a solid phase and a liquid phase, and divided roughly into a nematic phase, a smectic phase and a cholesteric phase. Display elements making use of a nematic phase are most widely used among them at present. A lot of display modes have so far been developed, and a dynamic scattering mode (DS mode), a guest-host mode (GH mode), a twisted nematic mode (TN mode), a super twisted nematic mode (STN mode), a thin film transistor mode (TFT mode), a ferroelectric liquid crystal mode (FLC mode), etc. are known.
A recent development in these fields has mainly been in miniaturization, reduction in power consumption and increase in a response speed of liquid crystal display elements as seen in portable telephones, and liquid crystalline compounds and liquid crystal compositions have been required to have a low threshold voltage and a low viscosity.
A threshold voltage (Vth) is a function of a dielectric anisotropy (xcex94xcex5) as shown by the following equation (Mol. Cryst. Liq. Cryst., 12, 57 (1970)):
Vth=xcfx80(K/xcex50xcex94xcex5)1/2
wherein K is an elastic constant, and xcex50 is a dielectric constant in vacuo.
As seen from the above equation, increasing xcex94xcex5 or decreasing K can be considered as a method for reducing Vth. However, it is still difficult to actually control K by conventional techniques, and a liquid crystal material having large xcex94xcex5 has generally been used to meet the requirement. Therefore, liquid crystalline compounds having large xcex94xcex5 have actively been developed. Viscosity is an element which affects a response speed of a liquid crystal molecule against an electric field, and a liquid crystalline compound having a low viscosity is preferably used in a large amount in order to prepare a liquid crystal composition showing a high speed response.
In recent years, liquid crystal display elements have become widely used in information terminals and portable games. These display elements are inevitably driven by batteries, and therefore, it is requested that they are driven at a low threshold voltage and have a low power consumption so as to be used for long time. Particularly in order to reduce a power consumption of an element itself, a reflective display element not requiring backlight has actively been developed recently. Liquid crystal compositions used for these reflective display elements are required to have a small refractive anisotropy as well as a low threshold voltage. Accordingly, it is important in this field to develop a liquid crystalline compound having a large dielectric anisotropy and a small refractive anisotropy as a liquid crystal material constituting the composition. The following compounds (13) and (14) (JP-A 2-233626) can be shown as a representative liquid crystal material for driving a display element at a low voltage, which is used for a liquid crystal display element of a TFT mode: 
wherein R represents an alkyl group.
Both the compounds (13) and (14) have a 3,4,5-trifluorophenyl group at the terminal of a molecule and are expected as a liquid crystal material for driving a display element at a low voltage. However, the compound (13) has a small dielectric anisotropy (xcex94xcex5=about 10) for use in the reflective display element described above, and the compound (14) has a satisfactory dielectric anisotropy (xcex94xcex5=about 12) but has a large refractive anisotropy of about 0.12, so that it is considered difficult to prepare a liquid crystal composition which can sufficiently satisfy the above requirements by using these compounds.
JP-A 10-204016 discloses the following compound (15) having a difluoromethyleneoxy group as a bonding group, which shows far larger dielectric anisotropy (xcex94xcex5) of about 14 than that of the above compound (13) while showing a clearing point, a refractive anisotropy and a viscosity equivalent to those of the compound (13), and which is expected as a liquid crystal material for driving at a low voltage in various TFT modes and also as a liquid crystal material for the reflective, display elements described above. 
In Formula, R represents an alkyl group.
JP-A 10-204016 discloses a process for producing the above compound having a difluoromethyleneoxy group as a bonding group, in which a corresponding ester derivative is converted to a thioester derivative with a Lawesson""s reagent (Fieser 13, 38) and the derivative is further fluorinated by reacting with HF-pyridine in the presence of an oxidizing agent according to a method disclosed in JP-A 5-255165 to produce the above compound. 
In Formula, R1xe2x80x2 is an alkyl group; rings A1xe2x80x2, A2xe2x80x2 and A4xe2x80x2 are a 1,4-cyclohexylene group or a 1,4-phenylene group; and Z1xe2x80x2 is a single bond or xe2x80x94CH2CH2xe2x80x94.
In the process described above, however, the thioesterification reaction with a Lawesson""s reagent results in unsatisfactory yield. HF-Pyridine is strongly corrosive and requires a specific facility in handling in the fluorination reaction, so that the process is not necessarily simple and efficient for producing a difluoromethyl ether derivative. Further, there has been no process but the above reported process, for producing a compound in which a substituent for a carbon atom of a difluoromethyleneoxy group is a cyclohexylene group.
As explained above, any simple and efficient process for producing a difluoromethyl ether derivative which shows various suitable physical properties as a liquid crystal compound has not been found yet, and therefore, not only a difluoromethyl ether derivative as a liquid crystal compound but also a simple process for producing the same have been desired to be developed.
An object of the present invention is to provide a process for producing a difluoromethyl ether derivative which shows various suitable physical properties as a liquid crystal compound, a liquid crystal composition comprising the compound and a liquid crystal display element containing the liquid crystal composition, and to provide a simple and efficient process for producing the above difluoromethyl ether derivative.
The present inventors have made intensive investigations in order to achieve the above object and, as a result, found that the intended difluoromethyl ether derivative can be obtained in a high yield by adding halogen to an xcex1,xcex1-difluorocyclohexylidene derivative or a difluorovinyl derivative as an intermediate, reacting with a phenol derivative, and then reducing it with hydrogen. Further, the inventors have found that when adding hydrogen halide to the difluorovinyl derivative, a halogen atom is quantitatively added onto a difluoromethyl carbon and that it is then reacted with a phenol derivative, whereby the intended difluoromethyl ether derivative can be obtained in a high yield. In addition, the inventors have found that the compound represented by Formula (1cxe2x80x2) having a 1,1-difluoro-2-propenyloxy group as a bonding group, which is obtained as an intermediate in the process of the present invention, has excellent characteristics as a liquid crystal material, and thus completed the present invention.
The present invention relates to:
a process for producing a difluoromethyl ether derivative represented by Formula (1d): 
wherein R1 and R2 each independently represent hydrogen, halogen, a cyano group or an alkyl group having 1 to 20 carbon atoms, in which at least one xe2x80x94CH2xe2x80x94 may be substituted with xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94 or xe2x80x94Cxe2x89xa1Cxe2x80x94 but xe2x80x94Oxe2x80x94 is not adjacent to another xe2x80x94Oxe2x80x94, and in which any hydrogen may be substituted with fluorine;
rings A1, A2, A3 and A4 each independently represent a 1,4-cyclohexylene group in which at least one xe2x80x94CH2xe2x80x94 may be substituted with xe2x80x94Oxe2x80x94 or xe2x80x94Sxe2x80x94, or a 1,4-phenylene group in which at least one xe2x95x90CHxe2x80x94 may be substituted with xe2x95x90Nxe2x80x94 and at least one hydrogen on the ring may be substituted with fluorine, a cyano group or an alkyl group having 1 to 10 carbon atoms;
Z1, Z2, Z3 and Z4 each independently represent a single bond or an alkylene group having 1 to 4 carbon atoms, in which at least one xe2x80x94CH2xe2x80x94 may be substituted with xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94 or xe2x80x94Cxe2x89xa1Cxe2x80x94 but xe2x80x94Oxe2x80x94 is not adjacent to another xe2x80x94Oxe2x80x94, and in which any hydrogen may be substituted with fluorine;
Y1, Y2, Y3 and Y4 each independently represent hydrogen, halogen, a cyano group or an alkyl group having 1 to 10 carbon atoms; and
k, l, m and n each independently represent 0 or 1;
which comprises using an xcex1,xcex1-difluorocyclohexylidene derivative represented by Formula (1a): 
wherein R1, rings A1 and A2, Z1, Z2, k and l have the same meanings as defined above; as a starting material.
The first aspect of the production process according to the present invention relates to the process described above, which comprises:
a first step of reacting a compound (1a) having an xcex1,xcex1-difluorocyclohexylidene group at the terminal with halogen to produce a compound (1b) 
wherein R1, rings A1 and A2, Z1, Z2, k and l have the same meanings as defined above, and X represents chlorine, bromine or iodine;
a second step of reacting the compound (1b) with a phenol compound (P) 
wherein R2, rings A3 and A4, Z3, Z4, Y1, Y2, Y3, Y4, m and n have the same meanings as defined above; in the presence of a base to produce a compound (1c) 
wherein R1, R2, rings A1, A2, A3 and A4, Z1, Z2, Z3, Z4, Y1, Y2, Y3, Y4, k, l, m and n have the same meanings as defined above; and
a third step of reducing the compound (1c) by hydrogenation to produce the compound (1d).
The second aspect of the production process according to the present invention relates to the process described above, which comprises:
a first step of reacting a compound (1a) having an xcex1,xcex1-difluorocyclohexylidene group at the terminal with hydrogen halide to produce a compound (1bxe2x80x2) 
wherein R1, rings A1 and A2, Z1, Z2, k and l have the same meanings as defined above, and X represents chlorine, bromine or iodine; and
a second step of reacting the compound (1bxe2x80x2) with the phenol compound (P) in the presence of a base to produce the compound (1d).
Further, the present invention relates to a process for producing a difluoromethyl ether derivative represented by the above Formula (1c) in the first aspect of the production process, which comprises:
a first step of reacting the starting material, an xcex1,xcex1-difluorocyclohexylidene derivative represented by Formula (1a) with halogen to produce the compound (1b); and
a second step of reacting the compound (1b) with a phenol compound (P) in the presence of a base to produce the compound (1c).
The present invention also relates to a process for producing a difluoromethyl ether derivative represented by Formula (1dxe2x80x2): 
wherein R1 and R2 each independently represent hydrogen, halogen, a cyano group or an alkyl group having 1 to 20 carbon atoms, in which at least one xe2x80x94CH2xe2x80x94 may be substituted with xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94 or xe2x80x94Cxe2x89xa1Cxe2x80x94 but xe2x80x94Oxe2x80x94 is not adjacent to another xe2x80x94Oxe2x80x94, and in which any hydrogen may be substituted with fluorine;
rings A1, A2, A3, A4 and A5 each independently represent a 1,4-cyclohexylene group in which at least one xe2x80x94CH2xe2x80x94 may be substituted with xe2x80x94Oxe2x80x94 or xe2x80x94Sxe2x80x94, or a 1,4-phenylene group in which at least one xe2x95x90CHxe2x80x94 may be substituted with xe2x95x90Nxe2x80x94 and at least one hydrogen on the ring may be substituted with fluorine, a cyano group or an alkyl group having 1 to 10 carbon atoms;
Z1, Z2, Z3 and Z4 each independently represent a single bond or an alkylene group having 1 to 4 carbon atoms, in which at least one xe2x80x94CH2xe2x80x94 may be substituted with xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94 or xe2x80x94Cxe2x89xa1Cxe2x80x94 but xe2x80x94Oxe2x80x94 is not adjacent to another xe2x80x94Oxe2x80x94, and in which any hydrogen may be substituted with fluorine;
Y1, Y2, Y3 and Y4 each independently represent hydrogen, halogen, a cyano group or an alkyl group having 1 to 10 carbon atoms;
k, l, m and n each independently represent 0 or 1; and o represents an integer of 1-10;
which comprises using an 1,1-difluorovinyl derivative represented by Formula (1axe2x80x2): 
wherein R1, rings A1, A2 and A5, Z1, Z2, k, l and o have the same meanings as defined above; as a starting material.
The third aspect of the production process according to the present invention relates to the process described above, which comprises:
a first step of reacting a compound (1axe2x80x2) having a 1,1-difluorovinyl group at the terminal with halogen to produce a compound (1bxe2x80x3) 
wherein R1, rings A1, A2 and A5, Z1, Z2, k, l and o have the same meanings as defined above, and X represents chlorine, bromine or iodine;
a second step of reacting the compound (1bxe2x80x3) with the phenol compound (P) in the presence of a base to produce a compound (1cxe2x80x2) 
wherein R1, R2, rings A1, A2, A3, A4 and A5, Z1, Z2, Z3, Z4, Y1, Y2, Y3, Y4, k, l, m, n and o have the same meanings as defined above; and
a third step of reducing the compound (1cxe2x80x2) by hydrogenation to produce the compound (1dxe2x80x2).
The fourth aspect of the production, process according to the present invention relates to the process described above, which comprises:
a first step of reacting the compound (1axe2x80x2) having a 1,1-difluorovinyl group at the terminal with hydrogen halide to produce a compound (1bxe2x80x2xe2x80x3) 
wherein R1, rings A1, A2 and A5, Z1, Z2, X, k, l and o have the same meanings as defined above; and
a second step of reacting the compound (1bxe2x80x2xe2x80x3) with the phenol compound (P) in the presence of a base to produce the compound (1dxe2x80x2).
Further, the present invention relates to a process for producing a difluoromethyl ether derivative represented by Formula (1cxe2x80x2) in the third aspect of the production process described above, which comprises:
a first step of reacting the starting material, a 1,1-difluorovinyl derivative represented by Formula (1axe2x80x2) with halogen to produce the compound (1bxe2x80x3); and
a second step of reacting the compound (1bxe2x80x3) with the phenol compound (P) in the presence of a base to produce a compound (1cxe2x80x2).
The liquid crystalline compound of the present invention is represented by Formula (1cxe2x80x2): 
wherein R1 and R2 each independently-represent hydrogen, halogen, a cyano group or an alkyl group having 1 to 20 carbon atoms, in which at least one xe2x80x94CH2xe2x80x94 may be substituted with xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94 or xe2x80x94Cxe2x89xa1Cxe2x80x94 but xe2x80x94Oxe2x80x94 is not adjacent to another xe2x80x94Oxe2x80x94, and in which any hydrogen may be substituted with fluorine;
rings A1, A2, A3, A4 and A5 each independently represent a 1,4-cyclohexylene group in which at least one xe2x80x94CH2xe2x80x94 may be substituted with xe2x80x94Oxe2x80x94 or xe2x80x94Sxe2x80x94, or a 1,4-phenylene group in which at least one xe2x95x90CHxe2x80x94 may be substituted with xe2x95x90Nxe2x80x94 and at least one hydrogen on the ring may be substituted with fluorine, a cyano group or an alkyl group having 1 to 10 carbon atoms;
Z1, Z2, Z3 and Z4 each independently represent a single bond or an alkylene group having 1 to 4 carbon atoms, in which at least one xe2x80x94CH2xe2x80x94 may be substituted with xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94 or xe2x80x94Cxe2x89xa1Cxe2x80x94 but xe2x80x94Oxe2x80x94 is not adjacent to another xe2x80x94Oxe2x80x94, and in which any hydrogen may be substituted with fluorine;
Y1, Y2, Y3 and Y4 each independently represent hydrogen, halogen, a cyano group or an alkyl group having 1 to 10 carbon atoms;
k, l, m and n each independently represent 0 or 1; and
o represents an integer of 1-10.
The liquid crystalline compound of the present invention is that of the above Formula (1cxe2x80x2), wherein the ring A5 is a 1,4-cyclohexylene group.
The liquid crystalline compound of the present invention is that of the above Formula (1cxe2x80x2), wherein both Y1 and Y3 are fluorine atoms, and both Y2 and Y4 are hydrogen atoms.
The liquid crystalline compound of the present invention is that of the above Formula (1cxe2x80x2), wherein both Y1 and Y2 are hydrogen.
The liquid crystal composition of the present invention comprises at least one liquid crystalline compound described above.
The liquid crystal composition of the present invention also comprises at least one liquid crystalline compound described above as a first component and at least one compound selected from the group A consisting of compounds (2), (3) and (4) as a second component: 
wherein R3 represents an alkyl group having 1 to 10 carbon atoms, in which any xe2x80x94CH2xe2x80x94 not adjacent to each other may be substituted with xe2x80x94Oxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94 and any hydrogen may be substituted with fluorine; X1 represents fluorine, chlorine, xe2x80x94OCF3, xe2x80x94OCF2H, xe2x80x94CF3, xe2x80x94CF2H, xe2x80x94CFH2, xe2x80x94OCF2CF2H or xe2x80x94OCF2CFHCF3; L1 and L2 each independently represent hydrogen or fluorine; Z5 and Z6 each independently represent xe2x80x94(CH2)2xe2x80x94, xe2x80x94(CH2)4xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94CF2Oxe2x80x94, xe2x80x94OCF2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94 or a single bond; rings A and B each independently represent a 1,4-cyclohexylene group, a 1,3-dioxane-2,5-diyl group, or a 1,4-phenylene group in which hydrogen may be substituted with fluorine; and ring C represents a 1,4-cyclohexylene group, or a 1,4-phenylene group in which hydrogen may be substituted with fluorine.
The liquid crystal composition of the present invention comprises at least one liquid crystalline compound described above as a first component and at least one compound selected from the group B consisting of compounds (5) and (6) as a second component: 
wherein R4 and R5 each independently represent an alkyl group having 1 to 10 carbon atoms, in which any xe2x80x94CH2xe2x80x94 not adjacent to each other may be substituted with xe2x80x94Oxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94 and any hydrogen may be substituted with fluorine; X2 represents xe2x80x94CN or xe2x80x94Cxe2x89xa1Cxe2x80x94CN; ring D represents a 1,4-cyclohexylene group, a 1,4-phenylene group, a 1,3-dioxane-2,5-diyl group or a pyrimidine-2,5-diyl group; ring E represents a 1,4-cyclohexylene group, a 1,4-phenylene group in which hydrogen may be substituted with fluorine, or a pyrimidine-2,5-diyl group; ring F represents a 1,4-cyclohexylene group or a 1,4-phenylene group; Z7 represents xe2x80x94(CR2)2xe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94CF2Oxe2x80x94, xe2x80x94OCF2xe2x80x94 or a single bond; L3, L4 and L5 each independently represent hydrogen or fluorine; and b, c and d each independently represent 0 or 1.
The liquid crystal composition of the present invention comprises at least one liquid crystalline compound described above as a first component and at least one compound selected from the group C consisting of compounds (7), (8) and (9) as a second component: 
wherein R6 and R7 each independently represent an alkyl group having 1 to 10 carbon atoms, in which any xe2x80x94CH2xe2x80x94 not adjacent to each other may be substituted with xe2x80x94Oxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94 and any hydrogen may be substituted with fluorine; rings G and I each independently represent a 1,4-cyclohexylene group or a 1,4-phenylene group; L6 and L7 each independently represent hydrogen or fluorine, but L6 and L7 are not hydrogen at the same time; and Z8 and Z9 each independently represent xe2x80x94(CH2)2xe2x80x94, xe2x80x94COOxe2x80x94 or a single bond.
The liquid crystal composition of the present invention comprises at least one liquid crystalline compound described above as a first component, at least one compound selected from the group A consisting of the compounds (2), (3) and (4) as a second component and at least one compound selected from the group D consisting of compounds (10), (11) and (12) as a third component: 
wherein R8 and R9 each independently represent an alkyl group having 1 to 10 carbon atoms, in which any xe2x80x94CH2xe2x80x94 not adjacent to each other may be substituted with xe2x80x94Oxe2x80x94 or xe2x80x94CHxe2x95x90CHxe2x80x94 and any hydrogen may be substituted with fluorine; rings J, K and M each independently represent a 1,4-cyclohexylene group, a pyrimidine-2,5-diyl group, or a 1,4-phenylene group in which hydrogen may be substituted with fluorine; and Z10 and Z11 each independently represent xe2x80x94Cxe2x89xa1Cxe2x80x94, xe2x80x94COOxe2x80x94, xe2x80x94(CH2)2xe2x80x94, xe2x80x94CHxe2x95x90CHxe2x80x94 or a single bond.
The liquid crystal composition of the present invention comprises at least one liquid crystalline compound described above as a first component, at least one compound selected from the group B consisting of the compounds (5) and (6) as a second component and at least one compound selected from the group D consisting of the compounds (10), (11) and (12) as a third component.
The liquid crystal composition of the present invention comprises at least one liquid crystalline compound described above as a first component, at least one compound selected from the compound group C consisting of the compounds (7), (8) and (9) as a second component and at least one compound selected from the group D consisting of the compounds (10), (11) and (12) as a third component.
The liquid crystal composition of the present invention also comprises at least one liquid crystalline compound described above as a first component, at least one compound selected from the group A consisting of the compounds (2), (3) and (4) and at least one compound selected from the group B consisting of the compounds (5) and (6) as a second component and at least one compound selected from the group D consisting of the compounds (10), (11) and (12) as a third component.
Further, the liquid crystal composition of the present invention comprises the liquid crystal composition described above, and further, at least one optically active compound.
The liquid crystal display element of the present invention comprises the liquid crystal composition described above.